Glucose transport in skeletal muscle, which is the rate.limiting step for glucose disposal, is activated by two separate pathways. One pathway is stimulated by insulin, the other by exercise. As the initial, insulin- independent increase in muscle glucose transport induced by exercise wears off, it is replaced by an increase in muscle insulin sensitivity that can last for days. The sensitivity of glucose transport to stimulation by contractions, hypoxia and other agents is also increased. The increase in the muscle insulin sensitivity is one of the most important health benefits of exercise, but, nothing is known regarding the mechanisms by which it is mediated. A major goal of this research is to elucidate some of the mechanisms involved in the exercise-induced increase in the sensitivity of muscle glucose transport to stimulation by insulin and other agents. One aim is to determine whether the increased sensitivity can be explained by translocation of a greater number of GLUT4 glucose transporters into the plasma membrane in response to the same submaximal stimulus, or whether an increase in the intrinsic activity of GLUT4 is responsible. Another aim is to determine whether the effects of a submaximal insulin stimulus on the phosphorylation state and/or activity of signal transducing proteins in the pathway by which insulin stimulates glucose transport are enhanced after exercise. Insulin resistance resulting in non.insulin.dependent diabetes, and other metabolic abnormalities that increase the risk of atherosclerosis, is serious health problem. We will use wats fed a high.fat diet as a model of abdominal obesity with insulin resistance, with the goal of determining the mechanisms responsible for the insulin resistance and how they are reversed by exercise and negative caloric balance. One of our aims is to determine whether the muscle insulin resistance can be explained by translocation of fewer GLUT4 into the plasma membrane in response to a given stimulus, and/or by a decrease in GLUT4 intrinsic activity. Another aim is to determine whether there is a reduction in the activity or the expression of signal.transducing proteins in the insulin- signaling cascade in muscles of fat-fed rats. A third aim is to determine whether the muscle insulin resistance is rapidly reversible b a) in vitro incubation, b) a caloric deficit, and c) exercise. We and others have found that a prolonged increase in glucose transport into muscle, resulting in high muscle glycogen levels, results in development of insulin resistance of glucose transport. Our third goal is to test the hypothesis that this insulin resistance is due to binding of GLUT4 to glycogen, resulting in decreased availability of GLUT4 for translocation by insulin. We have considerable evidence that the effect of exercise on glucose transport is mediated by an increase in cytosolic Ca2+.